1. Field of the Invention
The present invention relates to satellite broadcasting receivers and, more specifically to a satellite broadcasting receiver capable of switching and receiving signal radio waves from two broadcasting satellites.
2. Description of the Background Art
A conventional receiving equipment for satellite broadcasting basically includes: a reflection plate (reflection mirror) called a dish for receiving a signal radio wave transmitted from a broadcasting satellite in a geostationary orbit; a satellite broadcasting receiver receiving the signal radio wave reflected by the dish through a waveguide and performing low noise amplification on the received signal radio wave of 12 GHz band and then converting it to an intermediate frequency signal of 1 GHz to 2 GHz for output; and a tuner unit selecting and demodulating a desired signal band from the intermediate frequency signal supplied from the satellite broadcasting receiver through a signal cable. Generally, the satellite broadcasting receiver is fixed to the dish by means of a supporting arm, and the dish, the satellite broadcasting receiver and the supporting arm constitute an antenna system (so called a parabolic antenna). The antenna system is provided outdoors, and electric power and a control signal are supplied to the satellite broadcasting receiver through the signal cable from the tuner unit provided indoors.
Like the above described satellite broadcasting receiver, the apparatus having a function of low noise amplification and a function of conversion to an intermediate frequency is generally referred to as a low noise block down converter (LNB). Thus, in the following, such a satellite broadcasting receiver (excluding the waveguide portion) is simply referred to as an LNB.
Recently, a satellite broadcasting receiving system has been developed which can receive broadcasting signals from two broadcasting satellites in a geostationary orbit. FIG. 13 is a schematic block diagram showing two LNBs and a waveguide forming such a satellite broadcasting receiving system (not showing a tuner unit).
Referring to FIG. 13, an LNB 100 is provided to receive a broadcasting signal from one broadcasting satellite, and an LNB 200 is provided to receive a broadcasting signal from the other broadcasting satellite. LNBs 100 and 200 have identical structures. A waveguide for LNB 100 and a waveguide for LNB 200 are integrally formed as a waveguide 300.
As LNBs 100 and 200 have identical structures, only the structure of LNB 100 will be described.
Provided inside waveguide 300 for LNB 100 are probes (not shown) for detecting a horizontal polarization component H and a vertical polarization component V from a signal radio wave received from a corresponding broadcasting satellite.
Horizontal and vertical polarization components H and V detected by the probes (not shown) are supplied to a low noise amplifier (LNA) 100a of LNB 100. LNA 100a switches and selects signals obtained by amplifying horizontal and vertical polarization components H and V of the signal radio wave received from the corresponding broadcasting satellite, and further amplifies and supplies it to a band pass filter (BPF) 10b. The switching of the horizontal and vertical polarization components by LNA 100a is controlled by a power supply control circuit 10d. 
BPF 100b passes a signal at 12 GHz band, which falls within a receiving band for satellite broadcasting, and more specifically at a frequency band from 12.2 GHz to 12.75 GHz, and applies it to a microwave monolithic integrated circuit (MMIC) 100c. 
MMIC 100c functions as a mixing circuit mixing a local oscillator signal of 11.2 GHz output form an oscillator 100e and an output from BPF 100b, and converts the signal in the range from 12.2 GHz to 12.75 GHz output from BPF 100b to an intermediate frequency signal in the range from 1000 MHz to 1550 MHz. More specifically, MMIC 100c and oscillator 100e form a frequency converting circuit. It is noted that an operation current is supplied from power supply control circuit 100d to MMIC 100c and oscillator 100e. 
The intermediate frequency signal obtained by the frequency conversion of MMIC 100c is applied to a terminal 100f through a capacitor C, and then transmitted to a tuner unit (not shown) provided indoors through a signal cable (not shown). It is noted that the operation of power supply control circuit 100d is controlled by a control signal supplied from the above mentioned tuner unit (not shown) through the signal cable, terminal 100f and an inductor L.
As described above, the other LNB 200 has the same structure as the above described LNB 100, and therefore the satellite broadcasting receiving system in FIG. 13 as a whole can receive four different types of signal radio waves in total, i.e., a horizontal or vertical component from one or the other broadcasting satellite.
However, in the conventional satellite broadcasting receiving system shown in FIG. 13, two signal cables are required for connection of two LNBs 100 and 200 to the tuner unit (not shown). An improved structure where only one signal cable is required is disclosed for example in Japanese Patent Laying-Open No. 10-173562.
In the improved structure (not shown), a satellite selecting switch switching and selecting outputs from LNA 100a of one LNB 100 and from LNA 200a of the other LNB 200 is provided. In addition, one system of a circuit structure (a BPF, MMIC, oscillator, capacitor, output terminal) is provided in the subsequent stage, which is required for frequency conversion or the like.
Then, an output from one LNA that has been selected by the satellite selecting switch is supplied to a frequency converting circuit of the above mentioned one system. Thus, an intermediate frequency signal obtained by converting the signal radio wave from one or the other broadcasting satellite by a common frequency converting circuit is output from one output terminal. Accordingly, only one signal cable is necessary for supplying a signal to the tuner unit.
However, such an improved satellite broadcasting receiving system still suffers from the following problem.
More specifically, referring to FIG. 13, LNA 100a of one LNB 100 includes two amplifiers respectively amplifying horizontal and vertical polarization components H and V from one broadcasting satellite. LNA 200a of the other LNB 200 also includes two amplifiers respectively amplifying horizontal and vertical polarization components H and V from the other broadcasting satellite. As these four amplifiers are always set in the operation state, power consumption is large and a power supply of the tuner unit for supplying electric power to these amplifiers is also large in size.
In addition, a horizontal/vertical selecting switch is required for switching outputs obtained by amplifying horizontal and vertical polarization components H and V for each of LNAs 100a and 200a. Further, to reduce the number of signal cables to one, as described above, a satellite selecting switch is also required. A plurality of elements for switching are required for these switches, so that the parts number of the whole system increases.